This invention relates generally to the creation of broadband light sources and to the creation of broadband light sources for imaging applications, such as medical imaging and optical coherence tomography, in particular.
There are many industrial, medical, and other applications in which high resolution (generally less than 10 micrometers) measurement of distances, thicknesses, and optical properties of a biological or other sample is required. These applications include measurements of biological tissue, semiconductors, and other materials containing thin layers, as well as non-destructive testing of small structures inside integrated optical circuits, optical connectors, optical couplers, semiconductor lasers and semiconductor optical amplifiers. Medical applications include microsurgery, optical triangulation for in vivo diagnosis using endoscopes, endomicroscopes, retinoscopes, scanning confocal microscopes, and scanning laser ophthalmoscopes. Methods and apparatus for performing high resolution precision measurements on biological and other samples are described in U.S. Pat. Nos. 5,459,570 and 5,321,501, the entirety of each is incorporated by reference herein.
The light source for medical imaging, such as the light source used in the optical coherence tomography (OCT) systems, poses fundamental engineering challenges. OCT light sources must emit high output power into a single mode and have broad spectral bandwidths. Currently available inexpensive light sources typically fail to satisfy these two requirements. For example, the superluminescent diode (SLED), a popular light source of inexpensive OCT systems, can be tailored to produce high optical power at the expense of high spectral bandwidth, or broad spectral bandwidth at the expense of high optical output power. Physical limitations make the fabrication of broadband, high-power SLEDs impractical. The invention described herein overcomes these engineering problems common to prior art light sources.
In accordance with the invention, apparatus, systems and methods are provided for generating broadband light with high optical power and broad spectral bandwidth. A system for generating a source of broadband light includes, in one embodiment, a plurality of light sources such as SLEDs or edge-emitting luminescent diodes (ELEDs) whose outputs are combined in at least one combiner unit. The combiner unit combines light from a plurality of light sources into a single mode output beam. In one embodiment, the single mode output beam is the input beam to an interferometer for optical coherence tomography.
In another embodiment, a time-gain compensation (TGC) unit is in communication with the plurality of light sources. The TGC unit controls the plurality of light sources independently and simultaneously. In one embodiment, control of the plurality of light sources by the TGC unit is in synchrony with the variation in optical delay in the reference arm of the interferometer. The TGC unit may feature, for example, a modulated current regulator.
In another embodiment, the plurality of light sources of the broadband light source system have partially overlapping optical spectra.
Other features of the broadband light source system include one or more polarization adjusters in optical communication with the combiner. The polarization adjuster is a polarization loop in one embodiment that adjusts the direction of the polarization axis of each of the output light beams.
In another embodiment of the invention, the light sources are used in a dual-input interferometer comprising a pair of optical circulators.
Light sources of the broadband light source system may be SLEDs or ELEDs. The SLEDs may emit one or more peak wavelengths separated by a fixed interval.
The combiner, in one embodiment, is a truncated star coupler in optical communication with a grating and a plurality of focusing optics, such as lenses. The combiner combines the outputs of the light sources having source bandwidths substantially identical to the fixed interval.
The combiner of the broadband light source according to the invention may be further embodied in a polarization multiplexer or a wavelength division multiplexer. The system according to the invention may further comprise a non-polarization optical combiner.